The acceleration of ions to form energetic beams of ions has been accomplished both electrostatically and electromagnetically. The present invention pertains to sources that utilize electromagnetic acceleration. Such sources have variously been called plasma, electromagnetic, and gridless ion sources. Because the ion beams are typically dense enough to require the presence of electrons to avoid the disruptive mutual repulsion of the positively charged ions, the ion beams are also neutralized plasmas and the ion sources are also called plasma sources.
In ion sources (or, in space propulsion, thrusters) with electromagnetic acceleration, there is a discharge between an electron-emitting cathode and an anode. The accelerating electric field is established by the interaction of the electron current in this discharge with a magnetic field created between the anode and cathode. This interaction generally includes the generation of a Hall current normal to both the magnetic field direction and the direction of the electric field that is established. For the Hall current to be utilized efficiently, it must take place in a closed path within the discharge volume.
A Hall-current ion source can have a circular acceleration channel with only an outside boundary, where the ions are accelerated continuously over the circular cross section of this channel. This type of Hall-current ion source usually has a generally axial magnetic field shape as shown in U.S. Pat. No. 4,862,032—Kaufman et al, and as described by Kaufman, et al., in Journal of Vacuum Science and Technology A, Vol. 5, No. 4, beginning on page 2081. These publications are incorporated herein by reference.
A Hall-current ion source can also have an annular acceleration channel with both inner and outer boundaries, where the ions are accelerated only over an annular cross section. This type of Hall-current ion source usually has a generally radial magnetic field shape as shown in U.S. Pat. No. 5,359,258—Arkhipov, et al., and U.S. Pat. No. 5,763,989—Kaufman, and as described by Zhurin, et al., in Plasma Sources Science & Technology, Vol. 8, beginning on page R1. These publications are also incorporated herein by reference.
The cross sections of the acceleration channels are described above as being circular or annular, but it should be noted the cross sections can have other shapes such as an elongated or “race-track” shape. Such alternative shapes are described in the references cited. It should also be noted that the magnetic field shape can depend on the desired beam shape. For example, a radially directed ion beam would have a magnetic field generally at right angles to the magnetic field used to generate an axially directed ion beam.
There are inherent limitations of the Hall-current ion sources described above. One is the loss of neutral (unionized) gas that accompanies the generation of ions. The need for this loss can be described in a fairly simple manner. The ions are generated in a neutral plasma. During the time between the generation of a single ion and its departure from the region of generation, steady-state operation requires that a new ion be generated to replace it. At the same time, plasma neutrality requires that, on the average, only one electron is available to generate this replacement by ionizing a neutral atom or molecule. With the time for a single ionization by a single electron fixed, there is a minimum neutral density that will assure that the replacement ion is generated. The minimum neutral density to sustain the discharge results in a loss of neutral gas through the channel in which the ions are accelerated.
Another inherent limitation of a Hall-current ion source is the effect of background pressure on the maximum operating voltage, and hence on the maximum attainable ion energy. When the background pressure is significant, some of the neutral gas that is ionized comes from the backflow of neutrals from the background into the ion source. To compensate for this backflow at a given combination of discharge voltage and current, the external flow of neutral gas to the ion source must be reduced. The backflow thus results in an increase in neutral gas density near the exit plane and a decrease in neutral gas density near the anode, where the external flow of neutral gas is introduced. This shift in density distribution results in a corresponding shift in plasma density. More specifically, the reduction in plasma density near the anode reduces the ability of this plasma to sustain a discharge current. When the decrease in plasma density near the anode is sufficiently large due to the backflow of background gas, the discharge will at first fluctuate, or become “noisy,” and then will extinguish. The fluctuations in a noisy plasma are an aggravating factor in that they permit energetic electrons to more readily diffuse across the magnetic field and reach the anode, thereby being less effective in the generation of ions. In general, an increase in background pressure results in a decrease in the permissible maximum discharge voltage, and therefore the permissible maximum ion energy.
The escape of neutral gas and the effect of background pressure have serious adverse effects on ion source operation. The required pumping to sustain a given background pressure is increased by the loss of neutral gas. There is a necessary pumping that is required to offset the ion beam. That is, the ions will strike a target, recombine with electrons and become neutrals. The pumping must have sufficient capacity to carry away neutrals from these recombined ions and maintain the desired background pressure. The additional flow of neutral gas directly from the ion source adds to the required pumping capacity.
Sensitivity to background pressure can also add to the required pumping capacity. If two ion sources have the same ion beam currents and the same loss rate of neutral atoms or molecules of gas, the one that requires a lower background pressure for operation will also require more pumping capacity. To minimize the required pumping, it is desirable that an ion source tolerate a high background pressure.
Although space propulsion applications generally have negligible background pressure, the loss of neutral gas is serious and has a direct and adverse effect on overall efficiency.
The prior art summarized above all uses direct-current (dc) operation of ion sources. There have been limited departures from dc, or steady-state, operation in prior art. One departure has been short pulses when a very small amount of thin-film processing is required. Very short pulses have also been used in space propulsion when a very small impulse (the product of thrust times time) is required. Another departure from dc operation has been switching back and forth from one ion source to another to use multiple ion sources for thin-film processing while avoiding adverse interactions that might be encountered while operating two ion sources simultaneously. Yet another departure has been the use of quasisteady pulsed operation to determine the performance of an ion source or thruster with test facilities inadequate to sustain steady-state operation. In none of these prior-art departures from steady-state operation of Hall-current ion sources or thrusters have differences in electrical discharges been described compared to steady-state operation.